1. Field of the Invention
The present invention relates to a biaxial actuator for an optical pickup, which is used for recording a signal on and reproducing a signal from discs, such as a compact disc (CD), a mini-disc (MD), and a digital versatile disc (DVD). More particularly, the invention relates to a biaxial actuator for an optical pickup, which is enabled to improve the frequency characteristic by increasing the strength of a base member to which a lens holder is attached.
2. Description of the Related Art
FIG. 4 is an exploded perspective view illustrating a conventional biaxial actuator. This biaxial actuator is provided with a lens holder 1, which has an objective lens 11, a tracking coil 15, and a focusing coil 12, and a base member 102. The base member 102 has a pair of symmetrical rising pieces 123, 123 bend-formed at both sides of a bottom plate portion 122 having a beam passing aperture 121 so as to upwardly extend, a spindle portion 124 upwardly extending from the bottom plate portion 122, and a pair of projecting piece portions 125, 125 disposed in such a manner as to sandwich this spindle portion 124 from both sides thereof. A magnet 126 is attached to the inner surface of each of the pair of rising pieces 123, 123.
Further, a hole portion 13 provided in the lens holder 1 is fitted onto the spindle portion 124 of the base member 102 so that the spindle portion 124 can freely slide in an axial direction and rotate in a circumferential direction. Furthermore, a magnetic element is provided in the lens holder 1. This magnetic element is subjected to suction action of each of the right and left magnets 126, 126 provided on the base member 102. Consequently, the lens holder 1 is held in a floating state on an axial middle part of the spindle portion 124.
In this biaxial actuator, the lens holder 1 is controlled by electromagnetic action, which is caused by cooperation among the focusing coil 12, the tracking coil 15, and the magnets 126, in such a way as to displace in a focusing direction (that is, an axial direction) and in a tracking direction (that is, a rotating direction).
In the conventional biaxial actuator of this configuration, each of the left and right rising pieces 123 of the base member 102, to which the magnets 126 are attached, is formed so that a side elevation section thereof is rectangular, that is, the section extending from the top end 127 to the bend-formed root portion 128 has a same width over the whole height thereof, as shown in FIG. 5, and that the contours of the front and rear edges 129, 129 of each of the rising pieces 123 are constituted by straight lines that are perpendicular to the bottom plate portion 122.
On the other hand, recently, there has been promoted high increase in the recording density, at which information is recorded on an optical disc serving as a recording medium for audio visual equipment (AV equipment). Along with this, an optical pickup for reading the recorded information has been requested to improve the frequency characteristic thereof and to meet the demand for high increase in the recording density at which information is recorded on a disc.
Under such circumstances, hitherto, it has been known that the resonance frequency characteristic of an optical pickup is improved by increasing the stiffness of a moving part thereof, concretely, a lens holder. Further, it has been proposed as a concrete measure (see, for example, JP-A-11-66585) to glue a reinforcing member, which is made of a material, whose stiffness is higher than that of the lens holder, to the lens holder that serves as the moving part. Moreover, there has been proposed another measure (see, for instance, JP-A-8-194962), which increases the stiffness of the lens holder by setting the thickness of the root of an objective lens mounting portion of the lens holder. The root is protruded in a cantilevered shape.
However, in the conventional biaxial actuator described by referring to FIGS. 4 and 5, the rising pieces 123 of the base member 102, to which the magnets 126 serving as elements for controlling the lens holder 1 in such a way as to displace in a focusing direction (an axial direction) and in a tracking direction (a rotating direction) are respectively attached, are shaped so that the side elevation section of each of the rising pieces 123 is shaped like a rectangle, more concretely, a rectangle elongated in the direction of height thereof. It has been found that thus, the rising pieces 123 are apt to resonate when information is read from a disc on which information is recorded at a high recording density.
It has been considered useful for solving this problem to increase the stiffness thereof by applying ideas described in the aforementioned JP-A-11-66585 and JP-A-8-194962, that is, by gluing high-stiffness reinforcing members to the rising pieces 123, or by setting the rising pieces 123 to be thick.
However, in the case of forming the left and right rising pieces 123, 123 by bending both side portions in the width direction of the bottom plate portion 122 of the base member 102, as described with reference to FIG. 4 or FIG. 5, the thickness of the entirety of a metal plate used for manufacturing the base member 102 should be set to be thick so as to realize thick rising pieces 123. This causes a problem in that a steep rise in the manufacturing cost of the base member 102 occurs.